Electrical heaters

ABSTRACT

Electrical heaters having an outer metal sheath within which is a resistance element, conductors being in electrical contact with said element and projecting outwardly of said sheath for connection to a power source; said resistance element being surrounded by electrically insulating, heat-conductive material capable of withstanding relatively elevated temperatures, and hermetic seals provided at the ends of said sheath formed of swageable material capable of retention of inherent physical characteristics at substantially high temperatures.

United States Patent [72] lnventor Ronald M. Wrob Sunset Hills, M0. [21]Appl. No. 771,468 [22] Filed Oct. 29, 1968 [45] Patented June 1, 1971[73] Assignee Watlow Electric Manufacturing Co.

St. Louis, Mo.

|54| ELMF'IRICAL HEATERS 24 Claims, 1 1 Drawing Figs.

[52] U.S.(Jl .1 219/541, 73/359, 174/5052, 219/544, 219/553, 338/238,338/274 I51] Int. Cl 1105b 3/08 [50] Field olSearch 219/54l- 4, 553-2;338/23842, 237, 261, 273-4; 174/77, 50.52-50.59, 77 R; 123/145, 145 P;73/341, 359

[56] References Cited UNITED STATES PATENTS 1,857,614 5/1932 Backer338/238X 1,992,787 2/1935 Sutton 338/238X Primary Examiner- Volodymyr Y.Mayewsky Auumey- Robert J. Eck

ABSTRACT: Electrical heaters having an outer metal sheath within whichis a resistance element, conductors being in electrical contact withsaid element and projecting outwardly of said sheath for connection to apower source; said resistance element being surrounded by electricallyinsulating, heat-conductive material capable of withstanding relativelyelevated temperatures, and hermetic seals provided at the ends of saidsheath formed of swageable material capable of retention of inherentphysical characteristics at substantially high temperatures.

PATENTE DJuu nan v 8582,6516

' sum 1 or 2 I INVENTOR NALD M. WROB fir fw ATTORNEY ELECTRICAL HEATERSBACKGROUND OF THE INVENTION The present invention relates in general toelectrical heaters and, more particularly, to means for enhancing thehermetic properties of same.

A major problem in electrical heaters is the tendency of the resistanceelement to oxidize at high temperatures thereby resulting in asubstantial reduction of the operating life of the heater. It iswell-known in the art to hermetically seal the ends of the sheath so asto protect the resistance element from oxygen in the atmosphere, as inmoisture, or in gaseous form. For this purpose, ceramic seals have beengenerally used because of their relatively nonpourous, high temperaturere- .sistant character, but the same have proved inadequate because oftheir relative rigidity and brittleness which conducc frequently tofractures with development of open cracks which latter markedly increasethe leak rate of the seal. Accordingly, there has been the recognizedneed for relatively high temperature resistant hermetic seals of a moredurable and fracture resistant character which maintain their physicalcharacteristics at a temperature in excess of [000 F. and reliablyprevent the untoward entry of ambient oxygen within the sheath.

Additionally, the aforesaid problem has been compounded by reason of thenature of the customarily utilized insulating material in such heaterswhich is magnesium oxide. It appears that under conditions of relativelyhigh heat and low vapor pressures the magnesium oxide tends todissociate releasing oxygen resulting in loss of the insulatingproperties of the magnesium oxide and in the development of high leakagecurrents which further conduce to the breakdown of the insulatingmaterial. By reason of the foregoing, the selection of the metal orother materials forming the resistance element have been determined bytemperature considerations, with limitations below the desideratum.Accordingly, material such as graphite and refractory metals such asmolybdenum, niobium, and tungsten, which generate a markedly highertemperature than conventional resistance elements, could not beeffectively utilized in view of present heater technology. The extremesensitivity of said refractory metals to oxygen at high tempera tures iswell-known. Therefore, the industry has long sought an insulatingmaterial which may be utilized in place of magnesium oxide and whichwould permit of the effective incor poration in said heaters ofresistance elements from materials of the type stated whereby sheathtemperatures, not deemed hitherto normally obtainable, could beachieved.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a heater of the type stated having a hermetic seal constructedfrom a relatively gas impervious, swageable material which substantiallyincreases the heater life; which is extremely stable at hightemperatures; which permits maintenance of a constant high temperaturefor a long period of time; which possesses relatively great dielectricproperties; and which has substantial mechanical strength.

Another object of the present invention is to provide a heater of thetype stated, having a hermetic seal which is flexible and resilient sothat same can be subjected to rough han dling without fracturing orfragmenting thereby preserving its peculiar leak rate.

A further object of the present invention is to provide a heater of thetype stated, having a hermetic seal constructed of small oriented flakesof mica, such as mica paper.

A still further object of the present invention is to provide a heaterof the type stated, containing an insulating material with relativelylow gas porosity and low reactivity with refractory metals and graphiteand relatively high electrical and thermal properties so that oxygensensitive refractory materials may be used for the resistance elementsin such heaters so as to effect the development of relatively highsheath temperatures in both oxidizing and nonoxidizing atmospheres.

It is another object of the present invention to provide a heater of thetype stated, containing boron nitride as the insulating material and aresistance element formed from refractory materials having high meltingpoints and low vapor pressures, such as, molybdenum, niobium, tungsten,graphite and the like.

It is a further object of the present invention to provide a heater ofthe type stated, incorporating hermetic seals capable of reliablydenying entry of gases and particularly oxygenbearing gases from theatmosphere and insulating material having properties conducive to thedevelopment of relatively high temperatures in the resistance elementwithout diminu tion of the operating life ofthe heater.

Another object of the present invention is to provide a hermetic seal ofthe type stated for use within thermocouples for relatively increasingthe life of the latter by the development of a substantiallygas-impervious condition.

It is another object of the present invention to provide a heater of thetype stated which is inexpensive to manufacture and which is reliableand durable in usage.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspectiveview ofa tubular-typeheater having a hermetic seal constructed in accordance with andembodying the present invention.

FIG. 2 is a vertical longitudinal section taken on the line 22 of FIG.1.

FIG. 3 is an end view of the tubular heater shown in FIG. 1.

FIG. 4 is a side view of a tubular heater, having portions broken away,illustrating same in a partially swaged condition.

FIG. 5 is a perspective view ofa cartridge-type heater incorporatinganother embodiment of a hermetic seal constructed in accordance with andembodying the present invention.

FIG. 6 is a vertical longitudinal section taken on the line 6-6 of FIG.5.

FIG. 7 is an end view of the cartridge heater as shown in FIG. 5.

FIG. 8 is a vertical longitudinal section taken through a cartridge-typeheater incorporating another embodiment of a hermetic seal constructedin accordance with and embodying the present invention.

FIG. 9 is an end view of FIG. 8.

FIG. 10 is a side view ofa tubular-type heater, with portions brokenaway, incorporating another embodiment of the present invention.

FIG. 11 is a vertical longitudinal section taken through a thermocoupleincorporating a hermetic seal of the present invention.

DESCRIPTION OF THE PRACTICAL EMBODIMENTS Referring now by referencecharacters to the drawings which illustrate practical embodiments of thepresent invention, H generally designates a heater of the tubular-typehaving an outer metal sheath 1 within which is coaxially disposed aresistance element 3, shown in FIGS. 2 and 3 as a helical wire althoughother conventional forms, such as of rod, tubular, mesh and ribbonconstruction, may be readily substituted therefor; said element beingformed of conventional metal alloys such as of copper, nickel, chromiumand the like. The opposite ends of element 3, as at a, a, projectinwardly into opposite ends of a bore 4 of element 3 for suitableconnection to conductors 5, 5', respectively, for making electricalcontact therewith. The terminals of each conductor 5, 5 extend outwardlyfrom opposite ends of sheath 1 for connection to a suitable power source(not shown). Element 3, which is of less diameter and length thansheathe l, defines therewith an intervening annular space 6, which isfilled, along with bore 4, with a granular electrically insulating,heat-conductive refractory material, indicated 7, such as magnesiumoxide, for surrounding said element 3, and a compartment c adjacent eachend for accommodating a high-temperature, hermetic seal A beingresistant to temperatures in excess of l000 F.; there being a no heat"spacer 2, as of ceramic material, interposed between seal A and element3 so as to insulate the former from direct contact with the latter. Saidhermetic seal A is constituted of resilient, gas-impervious, swageablematerial, such as mica paper, which is wound upon itself into a roll 8.Both seal A and spacer 2 are provided with aligned axial openings, as at9, for extension therethrough of conductor 5, 5'.

Mica is a comprehensive term for certain complex hydrous aluminumsilicate compounds having a sheet or platy structure and being extremelyvariable in composition, containing metallic ions, particularly thosefrom the alkali group. Mica paper is flexible, consisting of line flakesof mica deposited as a continuous and highly uniform mat or sheet whichis durable and resistant to fragmentation, fracturing and the like dueto the overlapping laminar structure thereof. Mica paper is stable atrelatively high temperatures and has recognizedly substantial dielectricstrength and electrical resistivity. The physical characteristics ofmica paper are unaltered under heat, to a temperature of about l600 F.whereat the mica paper tends to expand, thereby enhancing its sealingproperties.

With rolls 8 of mica paper being positioned within compartment 0, heaterH is swaged throughout its length in a conventional manner, so that thediameter of same is decreased in the range of 10 percent to 25 per cent,as suggested in FIG. 4, thereby compressing rolls 8, insulating material7, and spacer 2 into a dense, compact unit; augmenting the airtightcharacter of heater H. lt has been found that by substituting tworelatively shorter mica paper rolls for each of the aforesaid rolls 8,the hermetic quality ofseal A is markedly increased as a result of thelabyrinth effect caused by the laminated structure of mica as well asthe non alignment of any pores that may be peculiar to one roll.Accordingly, it is within the contemplation of the present inventionthat seal A may comprise a single roll of would mica paper, asdesignated 8, or may consist ofa plurality of shorter rolls forestablishing the requisite gasimpervious condition.

Testing has revealed that mica paper roll seals have a consistent leakrate within the range of 10' to lo cm.'"/sec. which conduces to theenhancement of the effective life of heaters, such as H, utilizing saidseals. A leak rate of 10" cm. lsec. has been considered within thisfield a substantially airtight seal.

Referring now to FIGS. 5, 6, and 7 of the drawings, H generallydesignates a heater of the cartridge-type comprising a cylindrical metalsheath ll closed at one end, as at d, and having coaxially disposed acore 12 therein fabricated of insulating material, there being a helicalresistance element w coiled therearound. The end 13 of element w arebent for extension into the proximate end of parallel longitudinal bores14, 14' formed within core 12. Extending through each of said bores 14,14' is a conductor 15, 15' respectively which engage the opposite ends13, of resistance element w for establishing electrical contacttherewith. It will be understood that the form of element w a nd theparticular means for effecting electrical connection between same andconductors 15, 15 do not constitute part of the present invention asother conventional means may be readily substituted therefor. Theterminals of conductors l5, 15' project outwardly of the open end e ofsheath 11 for suitable connection to a power source (not shown). Core 12is of less diameter and length than sheath 11 and defines therewith anintervening annular space 16 which is filled with a granular,electrically insulating heat-conductive, refractory material 17, such asmagnesium oxide, and an endwise opening compartment b at the open end ofsheath 11 within which is fitted a high-temperature hermetic seal Bbeing resistant to temperatures in excess of 1000 F.; there being a noheat" spacer 10, as of ceramic material, interposed between seal B andcore 12 so as to insulate the former from direct engagement with theresistance element w. Seal B comprises a plurality of discs 18, 19, 20,21 arranged in tight, face-to-face abutting relationship, each having apair of aligned apertures, as at 22, 22 through which conductors 15,

15 extend. Discs 18 and 20 are constructed of mica paper while discs 19and 21 are formed from insulating material, such as magnesium oxide,lava or the like, although lava is preferred; and said discs arepresented in alternating relationship, with mica disc 18 beinginnermost, abutting against the proximate end face of spacer l0. Micapaper discs are stable at relatively high temperatures and possessrecognized dielectric strength and electrical resistivity immediatelycomparable to mica paper rolls 8 described hereinabove. Since mica paperis relatively hygroscopic, it is essential to insulate same againstreception of ambient moisture so as to assure maintenance of itselectrical resistivity. As is well-known, lava seals are relativelyimpervious to ambient moisture. Therefore, lava disc 19 is disposedbetween mica discs 18 and 20, and lava disc 21 is positioned outwardlyof disc 20, being exposed to the atmosphere, for protecting mica discs18 and 20 against undeserved collection of ambient moisture. It will beunderstood that any number of alternating mica and lava may be utilizedin seal B but it has been found that four such discs as arranged bringabout optimum results.

With discs 18-21 inclusive, being installed within the compartment b ofsheath ll, heater H is swaged throughout its length for reducing itsdiameter in the range of l0 per cent to 25 per cent causing acompression of said discs as well as the insulating material 17, spacerl0 and resistance element w. This swaging process causes heater H' to beformed into a compact unit which is substantially impervious to gas andre sistant to ambient moisture.

Discs 18 and 20 of seal 8 may be constructed of boron nitride, in lieuof mica paper, which compound being of normally powdered form provides arelatively high degree of airtightness after compacting through swaging.Lava discs, such as at 19, 21, are also utilized with boron nitridediscs in alternating or sandwiching relationship so as to provide thelateral stability to said boron nitride discs.

Tests have shown conclusively that cartridge heaters, such as H,utilizing seal B, have an effective life in excess of four times that ofcartridge heaters not using said seal.

Referring now to FIGS. 8 and 9, C generally designates a hightemperature hermetic seal disposed within the open end ofa cartridgeheater M which latter is identical in construction to the cartridgeheater H as shown in H6. 5, so that like numbers designate likeelements. Seal C comprises a metal plug 23 welded, brazed, or the like,as at m, to the adjacent annular end wall of sheath ll. Plug 23 isprovided with a pair of bores 24, 24' through which conductors 15, 15respectively extend for connection at their outer ends to a suitablepower source (not shown). The diameter of each bore 24, 24' is greaterthan the diameter of the respective conductor 15, 15 for receiving amica paper sleeve 25, 25 respectively in rolled form and disposedcoextensively within said bores of plug 23. Plug 23 is formed of thesame metal as sheath 11 and thus has the same expansion characteristicsso that there is obviated the development of any mechanical stresseswhich would nor mally be encountered through differentials in thermalexpansion if the said components were ofdifferent metals.

With seal C positioned within the open end of sheath ll, heater M isswaged throughout its length for compressing same together with seal C,insulating material 17, spacer l0 and core 12 to provide a compact unitwhich enhances the hermetic character of heater M, making samerelatively impervious to ambient gas. Although the hermetic seals A,B,and C of the present invention have been described for use withinelectrical heaters of the tubular and cartridge type as the preferredembodiment, it will be understood that said seals may be modified forincorporation within other type electrical heaters, such as flat,rectangular or the like.

As observed above, heretofore consideration had to be given totemperature limitations of tubular and cartridge heaters by reason ofthe characteristics of the particular materials used in forming theresistance elements. Most high temperature resistance materials willtend to effect a relatively rapid degeneration if the same are broughtinto contact with oxygen at a relatively elevated temperature so that ithas been necessary to make certain that the temperatures do not reachsuch damaging levels. Accordingly, materials such as molybdenum,niobium, tungsten, and graphite, which possess relatively high meltingpoints and low vapor pressures are extremely oxygen reactive at hightemperatures, have not been to the present time deemed practical for usein heaters of the type in question. Since magnesium oxide has been theinsulating material of choice, it is believed that conceivably thedissociation characteristics of such compound at elevated temperaturescause resulting of oxygen for interreaction with the metals with suchresulting in high leakage currents and resistance element embrittlementwith further deterioration in the physical properties of the insulatingmaterial. Accordingly, it has been discovered that by replacing themagnesium oxide with boron nitride as the insulating material, disposedsurroundingly of the resistance element within sheaths I, 11 thatelements formed from the aforesaid metals may be used without breakdownarid thereby allow of the development ofa temperature considerablyelevated above that and deemed heretofore maximum. Thus, in FIG. 10, Hindicates the tubular heater shown in FIG. 1 and with like referencenumbers designating like elements. 27 indicates insulating materialformed of boron nitride while W indicates a resistance element,helically wound upon itself, and being formed of one of the aforesaidhigh-temperature materials which can reach temperatures in the range of2600 to 3000 F. and be productive therefore of a sheath temperature ofapproximately 2400 F. in an oxidizing atmosphere.

Boron nitride possesses relatively low gas diffusivity and extremelyhigh thermal conductivity and electrical resistivity, and, whencompressed by swaging is markedly impervious to gas. By such swaging,the density of boron nitride is in excess of 2.0 grams per cubiccentimeter, and the air leak rate thereof is approximately cm./sec.Moreover, boron nitride acts as a getterer" for absorbing the smallvolume of oxygen that may penetrate the seals.

Tests have conclusively shown that heaters, such as H, utilizing micapaper rolls 8 as seals, boron nitride as insulating material 27 and oneof said high temperatures resistance materials, such as tungsten, havean effective life in excess of 10 times that of identical heaters whichare not subjected to the swaging operation.

Therefore, it can be seen that by the present invention the utilizationof mica paper and boron nitride as a seal in tubular and cartridgeheaters substantially increases the hermetic character as well as thelife span of the same. By substituting boron nitride for the commonlyused magnesium oxide, oxygen sensitive metals such as molybdenum,niobium, tungsten and graphite may be utilized for the resistanceelement resulting in developing considerably higher sheath temperaturethan heretofore believed attainable. Accordingly, a single tubular orcartridge heater may thus be created embodying both the unusual hermeticseals A, B and C of the present invention as well as boron nitride 27 asthe insulating material for resistance element with the heater thushaving properties of considerably greater magnitude than conventionalheaters.

The hermetic seals B and C of the present invention may also beincorporated within thermoelectric elements, such as thermocouples. Thelife of a thermocouple, or the duration of time over which thethermocouple retains acceptable accuracy of calibration, decrease withincreasing temperature and is affected by the ambient atmosphere. Attemperatures in excess of 1000 F. it is necessary to provide a hermeticseal therefor to offset the injurious effects of high temperatures in anoxidizing atmosphere or in any other atmosphere which tends to changethe composition of materialsthat are exposed to it. Referring now toFIG. 11 of the drawings, T generally designates a thermocouplecomprising a cylindrical metal sheath 31 being closed at one end, as at32, and having coaxially'disposed therein a core 33 fabricated ofinsulating material, such as magnesium oxide or aluminum oxide. Theoutside diameter of core 33 is substantially the same as the innerdiameter of sheath 3| for providing a snug fitting herein, the length ofcore 33 being less than that of sheath 31 for defining therewith acompartment 34 adjacent the open end of sheath 31. Formed within core 33are a pair of parallel, longitudinal bores 35, 35' for extensiontherethrough ofconductors 36, 36' respectively. Said conductors 36, 36'are fabricated from dissimilar metals, such as chromcl and alumel, andare joined at one end as by welding or the like, indicated at 37 knownin the art as the hotjunetion" which is received within a transverseslot 38 connecting the bores 36, 36' for facilitating flush abutment ofbore 33 against closed end 32. The terminals of con ductors 36, 36'project outwardly of the sheath open end for suitable connection to ameasuring device, (not shown) such as millivoltmcter or a potentiometercalibrated in temperature Thermocouples, such as T, are used to measuretempera ture by placing the hot junction 37 at the point of measurementwhile the terminals are connected to the measuring unit to complete thecircuit.

Provided for snug accommodation within compartment 34 is a hermetic sealB disposed adjacent core 33 and being of identical construction as thehermetic seal B described hereinabove. Said seal B is provided withaligned apertures, as at 39, 39', through which conductors 36, 36'extend. With seal B being installed within compartment 34, thermocoupleT is swagcd throughout its length for reducing its diameter in the rangeof l025 per cent causing a compression of seal B and core 33 foraccordingly forming thermocouple T into a compact unit which issubstantially impervious to gas and resistant to ambient moisture.

It will be understood that seal C, in addition to seal B, may be readilyinstalled within compartment 34 of thermocouple T for hermeticallyscaling the open end thereof in the same manner as above set forth withH, H'to heaters H,H' and M. By utilizing either of seals B or C withincompartment 34 ofa thermocouple T, the life of the latter is increasedby the development of a relatively, substantially gas-imperviouscondition.

Having thus described my invention, what I claim and desire to obtain byLetters Patent is:

l. A hermetic electrical device having a metal sheath; an electricalelement disposed within said sheath, said electrical element including apair of terminals projecting outwardly endwise of said sheath;electrically insulating, heat-conductive material surrounding saidelectrical element; a hermetic hightemperature seal provided within saidsheath surroundingly of said terminals, said seal being fabricated fromgas-impervious swageable material capable of resisting deformation attemperaturcs in excess of l,000 F., said material being selected fromthe class consisting ofmica paper and boron nitride.

2. A hermetic electrical device as defined in claim 1 and furthercharacterized by said seal comprising mica paper being of sheetcharacter and wound upon itself to form a roll, a mica paper roll beingpositioned in tight, surrounding engagement with each-terminal of saidelectrical element.

3. A hermetic electrical device as defined in claim 2 and furthercharacterized by there being at least a pair of mica paper rolls in endto end relationship disposed closurewise within said sheath in tightlysurrounding engagement with each terminal of said electrical element.

4. An electrical device as defined in claim 1 and further characterizedby said seal material comprising mica paper formed into a disc.

5. An electrical device as defined in claim 4 and further characterizedby a lava disc being presented on opposite sides of said mica paperdisc.

6. An electrical device as defined in claim 5 and further characterizedby at least one lava disc being presented on the outer face of said micapaper disc in abutting relationship,

7. An electrical device as defined in claim 1 and further characterizedby said seal material comprising boron nitride in disc form.

8. An electrical device as defined in claim 7 and further characterizedby a lavadisc being presented on opposite sides ofsaid boron nitridedisc.

9. An electrical device as defined in claim 7 and further characterizedby at least one lava disc being presented on the outer face of saidboron nitride disc in abutting relationship.

10. A hermetic electrical device as defined in claim 1 and furthercharacterized by said electrical element comprising an electricalresistance element electrical connected to said terminals saidelectrically insulating material comprising boron nitride.

11. An electrical device as defined in claim 10 and furthercharacterized by said hermetic seal being constructed of mica paperwould upon itself into roll form.

12. An electrical device as defined in claim 11 and furthercharacterized by said resistance element being selected from the groupconsisting of molybdenum, tungsten, niobium, and graphite.

13. An electrical device as defined in claim 10 and furthercharacterized by said hermetic seal consisting of micapaper formed intoa disc,'a lava disc being presented on the outer face of said mica paperdisc in abutting relationship thereto.

14. An electrical device as defined in claim 13 and furthercharacterized by said resistance element being selected from the groupconsisting of molybdenum, tungsten, niobium, and graphite.

15. An electrical device as defined in claim 10 and furthercharacterized by said hermetic seal comprising a first lava discdisposed within said compartment, a boron nitride disc inserted inabutting relationship to said first lava disc and a second lava discinserted within said compartment in abutting relationship to said boronnitride disc.

16. An electrical device as defined in claim 15 and furthercharacterized by said resistance element being selected from the groupconsisting of molybdenum, tungsten, niobium, and graphite.

17. An electrical device defined in claim 10 and further characterizedby said resistance element being selected from the group consisting ofmolybdenum, tungsten, niobium, and graphite.

18. An electrical device as defined in claim I and further characterizedby said insulating material comprising a core disposed within saidsheath, said core having a pair of parallel longitudinal bores extendingtherethrough, said electrical element comprising a first conductorextending through one bore of said core and a second conductor extendingthrough the other bore of said core, said conductors having inner andouter ends and being constructed of different metals, said conductorsbeing joined at their inner end, said outer ends comprising saidterminals.

19. An electrical device as defined in claim 18 and furthercharacterized by said hermetic seal comprising mica paper formed into adisc, and at least one lava disc being presented on the outer face ofsaid mica paper disc in abutting relationship.

20. An electrical device as defined in claim 18 and furthercharacterized by said hermetic seal comprising boron nitride in discform and at least one lava disc being presented on the outer face ofsaid boron nitride disc in abutting relationship.

21. A hermetic electrical device having a metal sheath provided with anopen end; an electrical element disposed within said sheath andincluding a pair of terminals projecting out wardly endwise through saidsheath open end; electrically insulating, heatconductive materialsurrounding said electrical element; a metal plug provided within saidsheath open end and having a pair of bores therein, said terminalsextending through said bores, the diameter of each of said bores beinggreater than the diameter of said terminals, a hermetic hightemperatureseal provided within each of said bores, said seal being fabricated fromgas-impervious swageable material, capable of resisting deformation attemperatures in excess of 1,000 F., said material consisting of micapaper wound upon itself to form a rolled-formed sleeve for dispositionwithin each plug bore surroundingly of the terminal of the resistanceelement.

22. A hermetic electrical device having a metal sheath provided with atleast one open end; an electrical element disposed within said sheathand including a pair of terminals projecting outwardly endwise throughsaid sheath open end; said electrical element consisting of anelectrical resistance element being electrically connected to saidterminals; an electrically insulating heat-conductive material providedwithin said sheath surroundingly of said resistance element; a metalplug provided within said sheath open end and having a pair of borestherein, said terminals extending through said bores, the diameter ofeach of said bores being greater than the diameter of said terminals; ahermetic, high-temperature seal provided within each bore, said sealbeing fabricated from gas-impervious swageable material, capable ofresisting deformation at temperatures in excess of l,000 F., saidmaterial consisting of mica paper wound upon itself to form a rolledform sleeve for disposition within each plug bore surroundingly of theadjacent terminal ofthe resistance element.

23. An electrical device as defined in claim 22 and furthercharacterized by said resistance element being selected from the groupconsisting of molybdenum, tungsten, niobium, and graphite.

24. A hermetic electrical device having a metal sheath provided with atleast one open end, a core of electrically insulating heat-conductivematerial disposed within said sheath, said core having a pair ofparallel longitudinal bores extending therethrough, an electricalelement including a pair of terminals, said electrical elementcomprising a first conductor extending through one bore of said core, asecond conductor extending through the other bore of said core, saidfirst and second conductors having inner and outer end portions andbeing constructed of different metals, said first and second conductorsbeing joined at their inner end portions, the outer end portions of saidfirst and second conductors projecting through said sheath open end andcomprising said terminals, a metal plug provided within said sheath openend and having a pair of bores therein, said terminals extending throughsaid bores, the diameter of each of said bores being greater than thediameter of said terminals, a hermetic, high-temperature seal providedwithin each bore, said seal being fabricated from gas-imperviousswageable material, capable of resisting deformation at temperatures inexcess of l,00O F., said material consisting of mica paper wound uponitself to form a rolledformed sleeve for disposition within each plugbore surroundingly of the terminal of said resistance element.

(2;;2?" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent3.582.616 Dated June i. 1971 Inventor(s) Ronald m. Wrob It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

[- In the Specification: .1

Column 2, line 69, erase "sheaths" and insert -sheath--.

Column 3, line 36 erase "would" and insert ---wound-,-

line 40, erase "10 to 10 and insert --l0" to l0 line 42, erase "10 andinsert -lO' l/ 6 Column 5, line 36, erase '10 3 and insert --l0 Column 6line 34, after "with" erase "H,H' (first occurrence) and insert-respect--.

In the Claims;

Column 6 Claim 6, line 67, erase 5" and insert --4-.

Column 7, Claim 10, line 6 erase "electrical" (second occurrence) andinsert ---electrically--;

line 7, after minals" insert Signed and sealed this 26th day of October1971.

(SEAL) Attest: A

ED'TJARD M.FLETCIIV1R, JR. ROBERT GOTTSCHALK Attestinp; Officer ActingCommissioner of Patents

1. A hermetic electrical device having a metal sheath; an electricalelement disposed within said sheath, said electrical element including apair of terminals projecting outwardly endwise of said sheath;electrically insulating, heat-conductive material surrounding saidelectrical element; a hermetic hightemperature seal provided within saidsheath surroundingly of said terminals, said seal being fabricated fromgas-impervious swageable material capable of resisting deformation attemperatures in excess of 1,000* F., said material being selected fromthe class consisting of mica paper and boron nitride.
 2. A hermeticelectrical device as defined in claim 1 and further characterized bysaid seal comprising mica paper being of sheet character and wound uponitself to form a roll, a mica paper roll being positioned in tight,surrounding engagement with each terminal of said electrical element. 3.A hermetic electrical device as defined in claim 2 and furthercharacterized by there being at least a pair of mica paper rolls in endto end relationship disposed closurewise within said sheath in tightlysurrounding engagement with each terminal of said electrical element. 4.An electrical device as defined in claim 1 and further characterized bysaid seal material comprising mica paper formed into a disc.
 5. Anelectrical device as defined in claim 4 and further characterized by alava disc being presented on opposite sides of said mica paper disc. 6.An electrical device as defined in claim 5 and further characterized byat least one lava disc being presented on the outer face of said micapaper disc in abutting relationship.
 7. An electrical device as definedin claim 1 and further characterized by said seal material comprisingboron nitride in disc form.
 8. An electrical device as defined in claim7 and further characterized by a lava disc being presented on oppositesides of said boron nitride disc.
 9. An electrical device as defined inclaim 7 and further characterized by at least one lava disc beingpresented on the outer face of said boron nitride disc in abuttingrelationship.
 10. A hermetic electrical device as defined in claim 1 andfurther characterized by said electrical element comprising anelectrical resistance element electrical connected to said terminalssaid electrically insulating material comprising boron nitride.
 11. Anelectrical device as defined in claim 10 and further characterized bysaid hermetic seal being constructed of mica paper would upon itselfinto roll form.
 12. An electrical device as defined in claim 11 andfurther characterized by said resistance element being selected from thegroup consisting of molybdenum, tungsten, niobium, and graphite.
 13. Anelectrical device as defined in claim 10 and further characterized bysaid hermetic seal consisting of mica paper formed into a disc, a lavadisc being presented on the outer face of said mica paper disc inabutting relationship thereto.
 14. An electrical device as defined inclaim 13 and further characterized by said resistance element beingselected from the group consisting of molybdenum, tungsten, niobium, andgraphite.
 15. An electrical device as defined in claim 10 and furthercharacterized by said hermetic seal comprising a first lava discdisposed within said compartment, a boron nitride disc inserted inabutting relationship to said first lava disc and a second lava discinserted within said compartment in abutting relationship to said boronnitride disc.
 16. An electrical device as defined in claim 15 andfurther characterized by said resistance element being selected from thegroup consisting of molybdenum, tungsten, niobium, and graphite.
 17. Anelectrical device as defined in claim 10 and further characterized bysaid resistance element being selected from the group consisting ofmolybdenum, tungsten, niobium, and graphite.
 18. An electrical device asdefined in claim 1 and further characterized by said insulating materialcomprising a core disposed within said sheath, said core having a pairof parallel longitudinal bores extending therethrough, said electricalelement comprising a first conductor extending through one bore of saidcore and a second conductor extending through the other bore of saidcore, said conductors having inner and outer ends and being constructedof different metals, said conductors being joined at their inner end,said outer ends comprising said terminals.
 19. An electrical device asdefined in claim 18 and further characterized by said hermetic sealcomprising mica paper formed into a disc, and at least one lava discbeing presented on the outer face of said mica paper disc in abuttingrelationship.
 20. An electrical device as defined in claim 18 andfurther characterized by said hermetic seal comprising boron nitride indisc form and at least one lava disc being presented on the outer faceof said boron nitride disc in abutting relationship.
 21. A hermeticelectrical device having a metal sheath provided with an open end; anelectrical element disposed within said sheath and including a pair ofterminals projecting outwardly endwise through said sheath open end;electrically insulating, heat-conductive material surrounding saidelectrical element; a metal plug provided within said sheath open endand having a pair of bores therein, said terminals extending throughsaid bores, the diameter of each of said bores being greater than thediameter of said terminals, a hermetic high-temperature seal providedwithin each of said bores, said seal being fabricated fromgas-impervious swageable material, capable of resisting deformation attemperatures in excess of 1,000* F., said material consisting of micapaper wound upon itself to form a rolled-formed sleeve for dispositionwithin each plug bore surroundingly of the terminal of the resistanceelement.
 22. A hermetic electrical device having a metal sheath providedwith at least one open end; an electrical element disposed within saidsheath and including a pair of terminals projecting outwardly endwisethrough said sheath open end; said electrical element consisting of anelectrical resistance element being electrically connected to saidterminals; an electrically insulating heat-conductive material providedwithin said sheath surroundingly of said resistance element; a metalplug provided within said sheath open end and having a pair of borestherein, said terminals extending through said bores, the diameter ofeach of said bores being greater than the diameter of said terminals; ahermetic, high-temperature seal provided within each bore, said sealbeing fabricated from gas-impervious swageable material, capable ofresisting deformation at temperatures in excess of 1, 000* F., saidmaterial consisting of mica paper wound upon itself to form a rolledform sleeve for disposition within each plug bore surroundingly of theadjacent terminal of the resistance element.
 23. An electrical device asdefined in claim 22 and further characterized By said resistance elementbeing selected from the group consisting of molybdenum, tungsten,niobium, and graphite.
 24. A hermetic electrical device having a metalsheath provided with at least one open end, a core of electricallyinsulating heat-conductive material disposed within said sheath, saidcore having a pair of parallel longitudinal bores extendingtherethrough, an electrical element including a pair of terminals, saidelectrical element comprising a first conductor extending through onebore of said core, a second conductor extending through the other boreof said core, said first and second conductors having inner and outerend portions and being constructed of different metals, said first andsecond conductors being joined at their inner end portions, the outerend portions of said first and second conductors projecting through saidsheath open end and comprising said terminals, a metal plug providedwithin said sheath open end and having a pair of bores therein, saidterminals extending through said bores, the diameter of each of saidbores being greater than the diameter of said terminals, a hermetic,high-temperature seal provided within each bore, said seal beingfabricated from gas-impervious swageable material, capable of resistingdeformation at temperatures in excess of 1,000* F., said materialconsisting of mica paper wound upon itself to form a rolled-formedsleeve for disposition within each plug bore surroundingly of theterminal of said resistance element.